Much depends on how many detection cells they have enabled (rows and columns), the sensitivity threshold to detect that motion, and how much motion is occurring during shooting. If one car has already triggered a shoot event, then the camera has to cycle and prepare itself for the next MD and shoot event. The time it takes to save a file and reset to detect motion. If there is already activity in the FOV after it comes out of its save-file and reset cycle then it will shoot immediately again after it is done saving the previous image. If there is no motion then it waits to detect some.
If you look at these frames you'll see a recurring theme going on. Either a car has just entered from either side, or there's one in the middle with another one at either edge. I suspect the one in the middle triggered one event from a previous frame, then the one at the edge triggered another event as the first one reached the middle. There are the few where there is a car alone in the center. This is most likely due to the camera already being busy saving and resetting, when ready the one already moving in the middle of the FOV will trigger the camera again. (edit) Notice too the wildly swinging trees and branches, and the abrupt changes from sunlight to clouds. All of these can trigger motion detection depending on how you set the sensitivity levels in the script parameters.
Some of the added benefits of the MD system is that you can set which rows and columns you want to use for detection and mask out all others. Had the originator of this sequence set the MD script parameters so only one small detection cell in the middle of his viewfinder was active, then only cars in the very center would have triggered an image, instead of what is occurring, any motion anywhere is triggering a shot.
Had they also used their camera's "Color Accent" mode if it has it, and only had it display red colors, all else being B&W, then they could also have set the script's "Measure Mode" (Y,U,V,R,G,B) to a value of 3. That causing it to detect only changes on the Red channel. Then only red cars in the middle would have triggered a shot. If you have RAW or DNG saving enabled then those files are unaffected by the "Color Accent" effect that is only applied to the JPG files.